Flexible electric circuit for heating comprising a metallised fabric

ABSTRACT

A flexible electric circuit, for example an electric heater, comprises a metallised fabric ( 14 ) the metal of which is photochemically etched to form the circuit ( 16 ), the fabric preferably being porous.

TECHNICAL FIELD

This invention relates to flexible electric circuits, and has particularthough not exclusive application to such circuits in the form of heatersand/or for incorporation in articles of clothing, footwear and fabricbased electrical devices.

BACKGROUND OF THE INVENTION

The manufacture of flexible heaters by photochemical etching metal foilbonded to thin, flexible electrical insulation materials is a wellestablished practice. FIG. 1 a shows metal foil 2 bonded to anelectrical insulation material 9 by the use of a suitable adhesive. Theresulting heater element 6 and termination pads 8 shown in FIG. 1 b areformed using conventional photochemical etching techniques. Leadwires 10(FIG. 1 c) are attached to the termination pads 8 by means of crimping,welding, soldering, conductive adhesives or other joining techniques.Electrical insulation is completed by bonding suitable insulationmaterial 12 on top of the etched heater element as shown in FIG. 1 d.

An alternative way to connect the leadwires 10 involves pre-formingopenings in the top insulation layer 12 which is then bonded to theetched heater element 6. Subsequently, the termination pads 8 of theheater element 6 are connected to leadwires or connectors usingcrimping, welding, soldering, conductive adhesives or other joiningtechniques. The electrical insulation is completed by covering theheater element termination and leadwire joint with a patch of insulatingmaterial using an appropriate adhesive.

The electrical insulation materials used are in sheet form (up to 1 mmthick) and are typically non-porous. Common types of flexible insulatingmaterials used are fibre reinforced silicone rubber, polyimide andpolyester. Metal sheet (typically 10 μm-500 μm thick) is bonded to theinsulating material using an adhesive. Metals and alloys used for heaterelements typically have a resistivity which has a low dependence ontemperature and include, for example, copper, nichrome, nickel andstainless steel. The resistance of the heater element, and consequentlythe operating temperature, is controlled by changing the type of metalfoil, the thickness of the metal foil or the heater element design.

Other types of flexible heaters available utilise different forms ofheating element and include wire-wound elements, interwoven carbon fibresheets and metallised synthetic fibre sheets such as nickel coatedpolyester.

It is also known to utilise metallised fabrics and similar meshstructures in the manufacture of flexible heaters, for example asdisclosed in GB 2,092,868 and DE 3210097. However such structures havetotal metallisation, and the electrical resistance is controlled by themetal composition, the density of application and the like.

SUMMARY OF THE INVENTION

It would be desirable to be able to provide a flexible electric circuitmore conveniently and economically manufactured than heretofore and inwhich the electrical characteristics, in particular the electricalresistance, can be more easily controlled than heretofore.

According to one aspect of the present invention there is provided aflexible electric circuit comprising a metallised fabric the metal ofwhich is photochemically etched to form the circuit.

It will be appreciated that such an arrangement is distinguished fromthe prior art in that the metal is modified by photochemical etching toprovide circuit elements of chosen configuration and electricalproperties.

Preferably the fabric is porous.

The fabric to be etched may be coated with a continuous layer of metal,for example by chemical reduction, by electro-deposition or bysputtering.

Alternatively the fabric may comprise yarns and/or fibres the individualyarns and/or fibres being encapsulated in metal prior to manufacture ofthe fabric.

According to a further aspect of the invention, there is provided amethod of manufacturing a flexible electric circuit comprising the stepsof providing a metallised fabric, and photochemically etching the metalto form the circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a to 1 d show the simplified manufacturing steps of a prior artflexible heater;

FIGS. 2 a to 2 d show the simplified manufacturing steps of a flexibleheater according to the invention;

FIG. 3 is a flow chart of the manufacturing steps of a flexible heateraccording to the invention;

FIG. 4 is a graph showing the log of the resistance change against trackwidth for a standard element design for various metal foil heaterelements and for the element of a heater according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2 a shows a sheet of heat resistant polymeric mesh 14 coated with acontinuous layer of metal and which forms the basis for a heateraccording to the invention.

The mesh 14 can take a variety of different configurations, a typicalcommercially available metallised woven polymeric mesh beingMetalester™. Such products are woven electroless nickel plated polyestermeshes with a variety of thread thicknesses, thread spacings, type ofweave and weight of nickel. Threads may typically have a diameter withinthe range 24 to 600 microns, a thread count of between 4 and 737 per cm,and a metal coating of varying weight per square metre.

The fabrics may be coated with a continuous layer of metal aftermanufacture, for example by sputtering, by chemical reduction or byelectro-deposition, which results in total encapsulation of all thethreads of the mesh in metal. In an alternative mesh, the individualwarp and weft threads may be metallised prior to fabric production, forexample by sputtering, by chemical reduction or by electro-deposition.

The fundamental novelty of the invention is that the metallised mesh isphotochemically etched to form the heater element, a typical element 16with termination pads 18 being shown in FIG. 2 b.

Crimp connectors or other suitable flexible substrate connection devices20 are fixed to the termination pads 18 allowing leadwires 22 to beattached as shown in FIG. 2 c, while electrical insulation is completedby bonding insulation material 24,26 to the top and bottom of the heateras shown in FIG. 2 d.

FIG. 3 shows in more detail the steps associated with the process ofmanufacturing an etched flexible heater from metallised woven fabric.

There are six main process steps involved, namely

-   1. design and generation of the phototool (boxes 28,30);-   2. material preparation (box 32);-   3. exposure and developing (box 34)-   4. etching (box 36)-   5. testing (box 38)-   6. finishing (box 40).

The material preparation step 32 is divided into the sub-steps of:

-   a) cutting the metallised fabric to length (box 42);-   b) mounting the cut fabric onto a hinged frame (box 44), typically    1.5 mm thick brown styrene board, to enable the otherwise flimsy    fabric to be more readily handled and to travel flat through the    subsequent multi-stage manufacturing process;-   c) cleaning the fabric with a commercial surface cleaning agent (box    46) to assist the adhesion of the photoresist—if the cleaning agent    is not used, and the surface is contaminated, there is a tendency    for poor adhesion of the photoresist to the metal to be etched which    can result in the etchant undercutting the photoresist and attacking    the metal area which forms the required image, in turn reducing the    track width and increasing the resistance of the track;-   d) applying the photoresist (box 48)—one method of applying the    photoresist is by dip-coating—i.e. immersing the fabric in a liquid    photoresist to ensure a controlled application of liquid photoresist    to all parts of the metallised polyester threads and to avoid    undercutting of the etched track due to non-application of    photoresist to parts of the threads.

The exposure and developing step 34 is divided into the sub-steps of:

-   a) cleaning the phototool (box 50);-   b) exposing the photoresist to ultraviolet light (box 52), and-   c) developing the image of the heater element on the fabric (box    54).

The etching step 36 is divided into the sub-steps of:

-   a) progressively etching away the unrequired metal (box 56), and-   b) removing the photoresist to leave the required heater element    (box 58).

The resultant flexible heater is then tested, for example by measuringthe electrical resistance, and by visual inspection (box 60).

The heater is finished by electrically insulating the porous wovenmetallised etched fabric, for example by bonding layers of suitableelectrically insulating sheet material to each side using a webconsisting of low melt fibres—adhesive in the open mesh area can beminimised by applying a vacuum during lamination—or by dip-coating orpaint-spraying the etched fabric with a suitable heat resistantlacquer—again the use of a vacuum after lacquer application willmaximise the mesh open area. To further improve the porosity of theheater, the laminated insulating material may be a micro-porousbreathable fabric or film. After lamination, the porous heater isprofiled by cutting to its final shape (box 62).

The attachment of leadwires and other components such as thermalprotection devices complete the product which is then re-tested forelectrical performance (box 64).

Clearly the desired electrical characteristics of a heater, and inparticular the heat output, will determine the particular metallisedwoven fabric to be used to manufacture the heater, and the width andlength of the element to be photochemically etched on the fabric.

FIG. 4 shows a graph of the log of the track width in mm against the logof the resistance of the track in ohms per foot for a standard elementdesign for each of four conventional etched foil heaters and a heateraccording to the invention.

The individual graphs in FIG. 4 are:

-   1. annealed nickel foil of 50 microns thickness;-   2. annealed copper foil of 18 microns thickness;-   3. annealed stainless steel (Grade 321) of 38 microns thickness;-   4. annealed copper foil of 35 microns thickness;-   5. Metalester™ MET 25/16 (25 g.m⁻²).

It can be seen that product 5, a heater according to the invention, hasa resistance substantially the same as that of the stainless steel foilheater 3.

More particularly, the metal, the fibre diameter, the thickness of themetal coating, the spacing of the fibres and the element design are alltaken into account to determine the required electrical characteristics.

In the above described example, the threads are typically polymeric,particularly polyester for relatively low temperature applications,although any synthetic or natural fibres may suffice as a base for theproduct of the invention.

In the invention the flexible electric circuit may be manufactured fromfabrics incorporating yarns of the following two classes:

-   -   (i) Continuous: mono or multi-filaments of indefinite length and        including polypropylene, polyethylene, chlorofibres, viscose        rayon, di- and tri-acetate, polyester, nylon, aromatic polyamide        (Nomex®), poly-paraphenylene terephthalamide (Kevlar®), and the        like. Nomex® and Kevlar® are registered trademarks of E.I. du        Pont de Nemours and Company.    -   (ii) Staple: fibres of definite length twisted, wrapped or        otherwise combined, including cotton, linen (flax), jute, wool,        mohair, cashmere, angora and other speciality hair fibres,        blends of varying composition thereof e.g. 60% cotton/40%        polyester and the like.        Fabrics made from the above yarns or fibres (depending on the        type of fabric structure) include and are not limited to the        following types:

-   (i) woven

-   (ii) nonwoven

-   (iii) knitted—includes warp and weft

-   (iv) composites—laminated structures incorporating but not limited    to the following: textiles, coatings, polymer films, membranes,    hydrophilic and micro-porous breathable films, metals, ceramics and    other materials.

-   (v) pressed felts

-   (vi) braids.

The metal is conveniently nickel, although any resistive metal could beused.

The resultant product is thin, flexible and porous, and can be producedrelatively inexpensively.

Flexible heaters according to the invention and in the form ofphotochemically etched metallised fabric mesh have a variety ofapplications, and can be incorporated in, for example, mosquito traps,wound care products such as medical bandages and dressings, surgicalmasks and visors, motorcycle visors, sports equipment visors, outdoorand performance clothing, footwear and articles to be moulded, and canbe used as aerospace de-icers. Other applications will be apparent tothose skilled in the art.

Although described above as heaters, the invention is equally applicableto flexible electric circuits for use other than as heaters.

Such a circuit is photochemically etched from metallised woven fabric asdetailed above with respect to the heater element 16, and any additionalcomponents that are required are mounted thereon. Such a thin, flexible,porous electric circuit can be embedded into articles of wearableclothing and footwear, for example outdoor and performance clothing,military clothing, medical and sports garments, ski and walking boots,trainers, or be incorporated into other products to enhance theirfunctionality and to enable the control of associated electricalequipment, for example, computers; computer keyboards; telephones;mobile telephones; personnel data organisers; computer mouse; personnelaudio; global positioning systems; domestic appliances; TV/videos; hi-fiand music systems; computer game consoles; electronic musicalinstruments; toys; lighting; clocks and watches; mosquito traps;personal healthcare products including heart rate and other vital signmonitors, disability and mobility aids; automotive user controls; sportsequipment; ski goggles; skis; crash helmets for motorcycles, scooters,bikes, snow sports, motor sports, water sports; sports braces; controlsfor wearable electronics; educational aids; medical applications such asbed pads and blankets; medical sensors; blood and glucose monitoringsensors and personal protection devices (including alarm systems).

1. A flexible electric circuit comprising a metallised fabric, whereinthe metal is photochemically etched to form the circuit.
 2. The circuitof claim 1, wherein the fabric is porous.
 3. The circuit of claim 1,wherein the fabric is coated with a continuous layer of metal.
 4. Thecircuit of claim 1, wherein the fabric comprises individual yarns, theindividual yarns being encapsulated in metal prior to manufacture of thefabric.
 5. The circuit of claim 1, wherein the fabric is selected fromthe group consisting of woven, non-woven, knitted, laminated composite,pressed felt, and braid fabrics.
 6. The circuit of claim 1, wherein thefabric is woven from polyester threads and the metal is nickel.
 7. Amethod of manufacturing a flexible electric circuit comprising the stepsof: providing a metallised fabric and photochemically etching the metalto form the circuit.
 8. The circuit of claim 1, wherein the fabriccomprises individual fibres, the individual fibres being encapsulated inmetal prior to manufacture of the fabric.